Howto VirtualBox VBoxManage increase number of cpus

$ vboxmanage list vms

$ Poweroff the VM

$ VBoxManage modifyvm my-vm-name --cpuhotplug on

$ VBoxManage controlvm my-vm-name plugcpu 0
$ VBoxManage controlvm my-vm-name plugcpu 1
$ VBoxManage controlvm my-vm-name plugcpu 2
$ VBoxManage controlvm my-vm-name plugcpu 3
$ VBoxManage controlvm my-vm-name plugcpu 4
$ VBoxManage controlvm my-vm-name plugcpu 5

$Poweron the VM

$ cat /proc/cpuinfo | grep processor | wc -l
6